This invention relates generally to display devices, and more particularly, to a method of concealing display element defects in a display device.
Various types of visual display devices are in widespread use to convert electrical signals into visual imagery. For example, there are emissive displays such as cathode ray tubes (CRT), light-emitting diode (LED) displays, field emission displays (FED), and gas discharge (plasma) displays. There are also non-emissive displays, such as liquid crystal displays (LCD), electrochromatic displays and electrocolloidal displays. The following discussion applies to all of these types of displays.
In a visual display device, the image is generally formed on a display surface of the device by selectively stimulating, through electronic means, discrete elements positioned on the display surface. The discrete display elements vary in light intensity and/or color in response to the applied electrical stimulation so that a plurality of adjoining areas of differing light intensity and/or color are obtained on the display surface. In particular, when the display elements are very small in comparison to the size of the display surface, a viewer of the display screen perceives a continuous image.
An active matrix liquid crystal display (LCD) is a good example of the utility of this display technology. The LCD generally includes a pair of opposed glass substrates that are bonded together to form an enclosed volume that is filled with a liquid crystal material. The interior surface of one of the substrates is continuously coated with a layer of a conductive material to form a display surface, while the interior surface of the opposing substrate is patterned into individual display element electrodes that are arranged in a matrix fashion. Associated with each display element electrode is a switching element, generally comprised of a Thin Film Transistor, or a Metal Insulator Metal diode. Selection or non-selection of the display elements is achieved by the switching operation of the switching elements to make the display operable. A back lighting system is generally located behind the substrate opposite the display surface to transmit light through the substrates and the liquid crystal material so that a luminous display is visible to the user.
Most commonly, two display modes are used in active matrix LCD displays. In the xe2x80x9cnormally blackxe2x80x9d mode, the back lighting system is shielded from the viewer by the liquid crystal when no signal is applied to a display element electrode. When a signal is applied to the display element electrode, light is transmitted through the display element, and is perceived by the viewer as an illuminated area. An alternative mode, referred to as the xe2x80x9cnormally whitexe2x80x9d mode, allows light to be transmitted when no signal is applied to the display element electrode, and correspondingly shields the back lighting system when a signal is applied to the display element electrode.
The switching elements, display element electrodes, and interconnections are generally patterned onto the substrate using well-known semiconductor fabrication methods to achieve a complex multilayered structure consisting of layers of semiconductor materials, insulating materials, and various types of metals. Although the frequency of fabrication defects occurring in the formation of the elements, electrodes, and interconnections is generally low, when an active matrix LCD includes several hundred thousand display elements, the probability that numerous switching devices, electrodes, and/or interconnections on the substrate will be abnormally formed is not insignificant. Consequently, the display elements associated with these defective structures will not operate as intended, and are generally perceived by the user as a display element that exhibits a constant white condition, or conversely, a constant black condition, depending on the mode of operation of the display. The presence of the foregoing display element defects may therefore degrade the image forming capability of the visual display to the point that it is unacceptable for end-use.
Presently, the high yield production of active matrix LCDs without display element defects presents a significant technical challenge. In general, displays are fabricated with some acceptable number of defects, which are corrected through the application of various techniques, in order to improve the yield of acceptable displays. In low-resolution displays, the usual yield inhibitor is the presence of only a moderate number of individual display element defects. In higher resolution arrays, where the contribution of an individual display element to the overall image is much less significant, the presence of array-line defects is of principal concern. Accordingly, considerable effort has been directed towards the development of techniques to correct or conceal display element defects of these types in order to salvage defective displays.
The correction of individual display element defects in active matrix LCD displays has received considerable attention. For example, U.S. Pat. No. 5,638,199 to Tsubota, et al. describes a repair method for a defective xe2x80x9cbright spotxe2x80x9d display element whereby a photosensitive resist is applied to the substrate on the display side of the device, followed by the projection of a light source through the display while all of the display elements in the display are activated. Locations in the display where the light projects through the display therefore correspond to defective display elements, and exposure of the photosensitive resist to the light source occurs at these locations. Upon development of the resist, an opaque layer is formed that conceals the xe2x80x9cbright spotxe2x80x9d defect. A particular shortcoming of this technique is that it introduces several new steps in the display fabrication procedure, and accomplishes only the opaque masking of xe2x80x9cbright spotxe2x80x9d defects.
Other prior art methods selectively alter the structure of the substrate surface in order to reorient the liquid crystal molecules so that a display element defect is concealed. For example, U.S. Pat. No. 5,926,246 to Tomita, et al. describes a method where the aligning film in a defective display element is irradiated by a laser to form minute grooves oriented in a direction that differs from the grooves that were originally present on the aligning film. As a result, the twist orientation of the liquid crystal molecule is altered, so that the light transmission qualities of the display element are permanently altered, thus making a xe2x80x9cbright spotxe2x80x9d defect in the display less conspicuous. A similar method is described in U.S. Pat. No. 5,636,042 to Nakamura, et al., wherein a series of randomly oriented grooves are formed on the aligning film by irradiation of the defective display element by a laser. The random aligning grooves permit the liquid crystal molecules to be oriented with a plurality of different twists and orientations thereby achieving an overall muting of a xe2x80x9cbright spotxe2x80x9d defect.
A significant shortcoming present in the foregoing methods is that the defective xe2x80x9cbright spotxe2x80x9d pixel must be individually corrected by selectively forming new aligning grooves in the aligning film to achieve the desired attenuation in illumination level. In a display containing a large number of display elements, correction of even a modest number of defective display elements will be a time consuming task, rendering these methods suitable for salvaging defective displays with only small numbers of display element defects.
Still other prior art methods seek to minimize the occurrence of defective display elements by providing redundant components during fabrication of the display. These prior art methods are directed in particular towards the correction of array-line defects referred to earlier. For example, U.S. Pat. No. 5,490,002 to Nicholas describes an active matrix device that uses two parallel-connected switching elements to provide fault tolerance in the event that a switching element is abnormally formed as an open circuit, while allowing a short circuited device to be removed by selective laser scribing.
These prior art methods have the disadvantage of requiring the selective correction of individual defective display elements through the application of laser scribing techniques to activate the redundant components, and to deactivate the defective components, which limits the practical applicability of such methods to displays with a small number of display element defects. Additionally, these methods may not correct instances where the display element failure has resulted from the failure to properly form other essential components of the display element, such as the electrode, or other interconnections, since these methods generally contemplate the failure of switching components only.
The invention is directed towards apparatus and methods for masking display element defects in a display device. In one aspect, an apparatus in accordance with the invention includes a display device that includes a plurality of display elements disposed on a display surface, at least one display element being at least partially defective, a translation unit that is coupled to the display device that is structured to impart a periodic motion to the display surface, a display signal source capable of providing input signals to the display device, and a control unit that is coupled to the translation unit and the signal source that controllably directs the movement of the display device and controllably directs the signal source to correspondingly shift the display signals. The shifted display signals, when displayed, conceal display element defects while presenting a stable image to a stationary viewer. In another aspect of the invention, the display unit is comprised of horizontal and vertical linear supports that permit the translation unit to translate the display in horizontal and vertical directions, the control unit is further comprised of horizontal and vertical actuators, and the control unit is further comprised of horizontal and vertical position sensors.